From b1ea585149ecd9663aae9607c1b1c508cc98aec2 Mon Sep 17 00:00:00 2001
From: Timothy Flynn <trflynn89@pm.me>
Date: Mon, 24 Jan 2022 14:42:51 -0500
Subject: [PATCH] LibC: Implement tzset with time zone awareness in accordance
 with POSIX

---
 Tests/LibC/TestLibCTime.cpp      | 64 ++++++++++++++++++++++++++++++++
 Userland/Libraries/LibC/time.cpp | 41 +++++++++++++++++---
 2 files changed, 100 insertions(+), 5 deletions(-)

diff --git a/Tests/LibC/TestLibCTime.cpp b/Tests/LibC/TestLibCTime.cpp
index fcf2327639..4fa829e2a2 100644
--- a/Tests/LibC/TestLibCTime.cpp
+++ b/Tests/LibC/TestLibCTime.cpp
@@ -10,6 +10,25 @@
 
 const auto expected_epoch = "Thu Jan  1 00:00:00 1970\n"sv;
 
+class TimeZoneGuard {
+public:
+    TimeZoneGuard()
+        : m_tz(getenv("TZ"))
+    {
+    }
+
+    ~TimeZoneGuard()
+    {
+        if (m_tz)
+            setenv("TZ", m_tz, 1);
+        else
+            unsetenv("TZ");
+    }
+
+private:
+    char const* m_tz { nullptr };
+};
+
 TEST_CASE(asctime)
 {
     time_t epoch = 0;
@@ -41,3 +60,48 @@ TEST_CASE(ctime_r)
 
     EXPECT_EQ(expected_epoch, StringView(result));
 }
+
+TEST_CASE(tzset)
+{
+    TimeZoneGuard guard;
+
+    auto set_tz = [](char const* tz) {
+        setenv("TZ", tz, 1);
+        tzset();
+    };
+
+    set_tz("UTC");
+    EXPECT_EQ(timezone, 0);
+    EXPECT_EQ(altzone, 0);
+    EXPECT_EQ(daylight, 0);
+    EXPECT_EQ(tzname[0], "UTC"sv);
+    EXPECT_EQ(tzname[1], "UTC"sv);
+
+    set_tz("America/New_York");
+    EXPECT_EQ(timezone, 5 * 60 * 60);
+    EXPECT_EQ(altzone, 4 * 60 * 60);
+    EXPECT_EQ(daylight, 1);
+    EXPECT_EQ(tzname[0], "EST"sv);
+    EXPECT_EQ(tzname[1], "EDT"sv);
+
+    set_tz("America/Phoenix");
+    EXPECT_EQ(timezone, 7 * 60 * 60);
+    EXPECT_EQ(altzone, 7 * 60 * 60);
+    EXPECT_EQ(daylight, 0);
+    EXPECT_EQ(tzname[0], "MST"sv);
+    EXPECT_EQ(tzname[1], "MST"sv);
+
+    set_tz("America/Asuncion");
+    EXPECT_EQ(timezone, 4 * 60 * 60);
+    EXPECT_EQ(altzone, 3 * 60 * 60);
+    EXPECT_EQ(daylight, 1);
+    EXPECT_EQ(tzname[0], "-04"sv);
+    EXPECT_EQ(tzname[1], "-03"sv);
+
+    set_tz("CET");
+    EXPECT_EQ(timezone, -1 * 60 * 60);
+    EXPECT_EQ(altzone, -2 * 60 * 60);
+    EXPECT_EQ(daylight, 1);
+    EXPECT_EQ(tzname[0], "CET"sv);
+    EXPECT_EQ(tzname[1], "CEST"sv);
+}
diff --git a/Userland/Libraries/LibC/time.cpp b/Userland/Libraries/LibC/time.cpp
index 83f8f89156..53cdd4d5c6 100644
--- a/Userland/Libraries/LibC/time.cpp
+++ b/Userland/Libraries/LibC/time.cpp
@@ -11,7 +11,9 @@
 #include <LibTimeZone/TimeZone.h>
 #include <assert.h>
 #include <errno.h>
+#include <limits.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/time.h>
 #include <sys/times.h>
@@ -358,15 +360,44 @@ long altzone;
 char* tzname[2];
 int daylight;
 
+static char __tzname_standard[TZNAME_MAX];
+static char __tzname_daylight[TZNAME_MAX];
 constexpr const char* __utc = "UTC";
 
 void tzset()
 {
-    // FIXME: Here we pretend we are in UTC+0.
-    timezone = 0;
-    daylight = 0;
-    tzname[0] = const_cast<char*>(__utc);
-    tzname[1] = const_cast<char*>(__utc);
+    // FIXME: Actually parse the TZ environment variable, described here:
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08
+    StringView time_zone;
+
+    if (char* tz = getenv("TZ"); tz != nullptr)
+        time_zone = tz;
+    else
+        time_zone = TimeZone::current_time_zone();
+
+    auto set_default_values = []() {
+        timezone = 0;
+        altzone = 0;
+        daylight = 0;
+        tzname[0] = const_cast<char*>(__utc);
+        tzname[1] = const_cast<char*>(__utc);
+    };
+
+    if (auto offsets = TimeZone::get_named_time_zone_offsets(time_zone, AK::Time::now_realtime()); offsets.has_value()) {
+        if (!offsets->at(0).name.copy_characters_to_buffer(__tzname_standard, TZNAME_MAX))
+            return set_default_values();
+        if (!offsets->at(1).name.copy_characters_to_buffer(__tzname_daylight, TZNAME_MAX))
+            return set_default_values();
+
+        // timezone and altzone are seconds west of UTC, i.e. the offsets are negated.
+        timezone = -offsets->at(0).seconds;
+        altzone = -offsets->at(1).seconds;
+        daylight = timezone != altzone;
+        tzname[0] = __tzname_standard;
+        tzname[1] = __tzname_daylight;
+    } else {
+        set_default_values();
+    }
 }
 
 clock_t clock()